Grapefruit

Grapefruit is on this site because it is one of the very few foods that appears in prescribing information with a number attached.

What it does

Grapefruit inhibits an enzyme in the gut wall and liver that breaks down a long list of medicines. Inhibiting it means less of the drug is destroyed before it reaches the bloodstream, so the same dose produces a higher level.

The effect is on the medicine, not on you. Nothing about grapefruit is harmful; it changes the arithmetic of a dose that somebody calculated on the assumption you were not drinking it.

That direction matters. Too much drug is a different problem from too little, and it is the one that produces side effects rather than treatment failure.

The numbers that exist

For atorvastatin, the NHS advises that you can eat normally “but avoid having too much grapefruit juice as it can increase the chance of side effects”, and gives a limit: “Do not have more than 1 or 2 small glasses of grapefruit juice a day.”

For warfarin the advice is stricter and unqualified: “do not drink grapefruit juice”, listed alongside “do not have cranberry juice or cranberry products”.

Two medicines, two different answers, which is the whole reason this cannot be reduced to a rule about grapefruit. The interaction depends on the drug.

Where it hides on a label

Whole fruit and juice are obvious. The rest is not.

Grapefruit juice in blends. Mixed citrus and breakfast juices often contain it without leading with it, and the front of the pack rarely says so.

Pomelo and some hybrids. Related citrus can carry the same compounds. Bitter orange is another one that turns up in supplements.

Grapefruit seed extract is a different matter and worth separating. It is sold as a supplement ingredient for unrelated reasons, and it is not the same thing as juice; the evidence about it is its own subject and mostly not about this interaction.

Reading a juice carton’s ingredient list rather than its front is the practical version of this page, and it is the same habit as everything on the page about reading labels when you take a prescription.

The mirror image

St John's wort acts on the same machinery in the opposite direction. Grapefruit inhibits the enzyme, so a drug level goes up. St John’s wort induces it, so a drug level goes down.

One risks side effects and the other risks the medicine quietly not working. Neither carries a warning on the product you buy, and one of them is a fruit.